Chemistry Reference
In-Depth Information
One should not mistake an acceleration of the polymerization reaction due to a rise in the
temperature under nonisothermal conditions for a true gel effect from a rise in viscosity. The gel
effect can occur when the temperature of the reaction is kept constant.
A critical analysis of the gel effect suggests that the situation is complicated. In some
polymerizations, three different stages appear to be present when
R
P
/[M][I]
1/2
is plotted against
conversion or against time [
95
]. The plot indicates that during the first stage there is either a constant
or a declining rate and during the second stage there is autoacceleration. During the third stage, there
is again a constant or a declining rate [
95
].
Numerous publication made a substantial case for associating and/or attributing the gel effect to
entanglement of polymerizing chain radicals, resulting in a marked reduction in the termination rate
parameter,
k
T
. This was often done by using the assumption that in the neighborhood of the gel
effect
k
T
is controlled by polymer self-diffusion, which in turn exhibits entangled polymer dynam-
ics. O'Neil et al. [
96
], however, argued against that opinion. They carried out a series of
experiments involving bulk polymerizations of methyl methacrylate and styrene and feel that
their data contradicts this widely held belief that the gel effect onset is related to the formation of
chain entanglements. The experimental conditions used were such that they tended to delay or
eliminate the formation of chain entanglement. These conditions were high initiator and/or chain
transferring agent concentrations and additions of low molecular weight polymers prior to the
reactions. The results indicated that the gel effect occurs readily in the absence of entanglement and
that delaying the onset of entanglements does not necessarily delay the onset of the gel effect. Also,
critical examination of the molecular weights produced in these experiments indicated values that
were too low for entanglement formation in solution (polymer plus monomer) and sometimes
even in bulk polymer, not only at the onset but also throughout the gel effect [
96
]. O'Neil et al.
[
96
], found that even under conditions where entanglements are likely to exist, the gel effect onset
does not correlate with polymer molecular weight of the chains produced in a manner consistent
with entanglement arguments. Whether the kinetics during the gel effect may be affected by
entanglements was left uncertain.
3.4.5 Polymerization of Monomers with Multiple Double Bonds
Polymerizations of monomers with multiple double bonds yield products that vary according to the
locations of these bonds with respect to each other. Monomers with conjugated double bonds, such as
1,3-butadiene and its derivatives, polymerize in two different ways. One way is through one of the
double bonds only. Another way is through both double bonds simultaneously. Such 1,4 propagation
is attributable to the effect of conjugation and hybridization of the C
2
-C
3
bond that involves
2
hybrid orbitals [
97
]. All three modes of propagation are possible in one polymerization reaction so
that the product can, in effect, be a copolymer. The 1,4, 1,2, and 3,4, placement in propagations can be
illustration as follows:
sp
R
2
R
2
R
4
R
4
R
1
n
R
5
R
R
1
R
5
R
R
3
R
3
1,4-placement
